Correlation between Drying Behaviors of Brown Coal and Its Pore Structures

摘要

Drying behaviors of brown coal are affected by internal water diffusion, which is controlled by pore structures. To diminish the effect of functional groups on drying, drying behaviors and pore structures of only one kind of coal dried at different heating rates and atmospheres were investigated. The final drying temperature was kept constant. The relationships between pore structures, including specific surface area (S-BET), volume of mesopores (V-meso), volume of macropores (V-macro), volume of total pores (V-total), and two fractal dimensions (D-I and D-II), and drying characteristics, including maximum drying rate (v(max)), activation energy of drying surface water (E-1), activation energy of drying pore water (E-2), apparent diffusion coefficient surface water (Deff-1), and apparent diffusion coefficient pore water (Deff-2), were correlated. The relationship between basic pore parameters and fractal dimensions was examined. For coals dried in N-2, the S-BET was confirmed as one of the key factors influencing D-I. The v(max) increased with heating rates, resulting from the higher temperature at the same drying time and larger S-BET and V-macro. The E-1 was higher at faster heating rates, because of the higher temperature gradient and the bigger D-I. The E-2 increased as heating rates increased from 3 to 20 degrees C/min and then decreased for flash drying (directly dried at 200 degrees C). The increase could be also due to the higher temperature gradient at higher heating rates. The decrease could be because of the effect of V-macro on Deff-2. For coals dried in different atmospheres, the higher E-1 and E-2 in air were due to oxidation reaction. The relatively large heat conductivity of N-2 led to the lower E-1. The small molecular diameter of CO2 led to the lower E-2. There was no consistent relationship between S-BET and D-I and between pore structures and drying characteristics for coals dried in different atmospheres.